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Microstructures and deformation mechanisms of ballistic impacts in stone

Campbell, Oliver 2022. Microstructures and deformation mechanisms of ballistic impacts in stone. PhD Thesis, Cardiff University.
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Bullet impacts are a ubiquitous form of damage to the built environment arising from armed conflicts. The risk of damage to cultural heritage is increasing as armed conflicts shift towards more urbanised centres, yet the effects and mechanisms of small arms damage are unclear. The aim of this thesis is to investigate the damage morphology and mechanisms of bullet impacts into stone. The influences of target materials, impact angle, and projectile type on damage in limestone and sandstone, was investigated through a series of controlled experiments. A proof of concept study showed that digital documentation can be an effective method for quantifying the surface morphology of impact damage, and that surface and subsurface damage are linked. Controlled firearm impacts were undertaken to study the influence of projectile type, target material, and angle of impact on surface and subsurface damage. Structure from motion photogrammetry produced 3D models of damage for quantitative analysis. The results showed that bullet impacts into natural stone targets at engagement distances of approximately 200m cause approximately conical craters, with some impacts having a complex two-part structure. Radial fractures centred on the impact crater are commonly present, and can reach sides adjacent to the target face. Oblique impact trajectories result in asymmetric crater morphologies that can be distinguished from damage caused by perpendicular trajectories. In the subsurface, damage is characterised by grain crushing and pore space collapse close to the impact point, with discrete open fractures characterising damage further away. Both surface and subsurface fractures have circum- and transgranular pathways close to the impact, but with increasing distance tend towards predominantly circumgranular pathways. Fracture intensity analysis reveals high fracture intensities close to the impact, which reduce beyond a distance of 10 mm from the impact. Conical fractures, or zones of increased fracture intensity, dip away from the crater centre, with the potential to form interconnected networks in the subsurface. Cratering from bullet impacts into stone is controlled by momentum transfer, meaning target and projectile properties have a strong influence on damage morphology. Limestone targets exhibited wider, deeper, and more voluminous craters than impacts in sandstone targets . Craters caused by a 5.56 × 45 mm NATO projectile, containing a steel tip, were wider and deeper than craters caused by a 7.62 × 39 mm, without a steel tip, under comparable conditions.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Earth and Environmental Sciences
Funders: The Leverhulme Trust
Date of First Compliant Deposit: 8 September 2023
Last Modified: 11 Sep 2023 10:34

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